em.scss

@function em($px, $context-or-rem: strip_unit($base-font-size)) {
  @if($context-or-rem == true) {
    @return $px / strip_unit($base-font-size) * 1rem;
  } @else {
    @return $px / $context-or-rem * 1em;
  }
}

/// Remove the unit of a length
/// @param {Number} $number - Number to remove unit from
/// @return {Number} - Unitless number
@function strip_unit($number) {
  @if type-of($number) == 'number' and not unitless($number) {
    @return $number / ($number * 0 + 1);
  }

  @return $number;
}